This invention relates to superconductive metal oxide ceramics, in particular to YBa.sub.2 Cu.sub.3 O.sub.x, wherein x equals from 6.5 to 7.2
The technology of superconductivity, particularly metal oxide ceramics which conduct electricity with no resistance at temperatures above the boiling point of liquid nitrogen, unlike previously known materials that can superconduct only near absolute zero, has been rapidly developing. These materials, i.e. certain metal-oxide ceramics, can conduct electricity with no resistance at temperatures above the boiling point of liquid nitrogen (77 K or -196.degree. C). The discovery of these materials has been quite recent, and the demand for practical application of these materials will be ever increasing in the future. In particular, it is desirable to develop these new metal-oxide ceramics, particularly YBa.sub.2 Cu.sub.3 O.sub.x, wherein x equals from 6.5 to 7.2 into useful conductor shapes such as wires, films, or thin sheets. It has not heretofore been possible to develop large, i.e. up to 1 mm in size, crystals of the material. It is desirable to do so for several reasons. Amongst those reasons include the ease of continued investigation of large crystals. The possibility that such large crystals can be easily developed into useful shapes such as superconducting rods, rings, wires, tapes and thin films.
Accordingly, this invention has as its primary objective the development of YBa.sub.2 Cu.sub.3 O.sub.x large single crystals.
Another objective of this invention is the development of such crystals by a process easy in practice and efficient in use from the standpoint of developing high yields of large, i.e. up to a millimeter or even larger size, crystals.
Perhaps one of the most promising of the new metal oxide ceramics found to be superconductors at relatively high temperatures is YBa.sub.2 Cu.sub.3 O.sub.x. Recent testing has shown that this superconducting ceramic is a distorted oxygen deficient form of perovskite with a layered structure not seen in naturally occurring minerals.
In the typical process for development of this ceramic used to date, there is heating of an intimate mixture of the oxide or the carbonate powders of the solid elements at temperatures between 900.degree. C. and 1100.degree. C. in order to drive off carbon dioxide and other volatiles. After regrinding and reheating, the mixture is pressed into pellets and sintered (bonded without melting) at high temperatures for several hours. The pellets are then annealed at low temperatures, optimally 400.degree. C. to 450.degree. C.
The conditions under which the ceramic is prepared affect the oxygen content. This is important because structural studies have shown that the number and arrangement of oxygen atoms in the lattice is a key determinate of the oxide superconductive properties. In fact, to produce the highest superconductive transition temperatures, the ceramic should be heated in an atmosphere that has some oxygen present, some workers have even said pure oxygen. For further details of superconductivity of YBa.sub.2 Cu.sub.3 O.sub.x ceramic, see Chemical Engineering and News, May 11, 1978, pp. 7-16.